Chemical vapor deposition (CVD) of metal siloxides has been used to deposit stable thin-film silicates as gate dielectrics (high dielectric constant) onto silicon substrates for use in the microelectronics industry. Single source metal siloxide precursors greatly simplify production and processing. Representative single source precursors which have been used in the past include Zr(OSiMe3)4, Zr(OSiEt3)4, Hf(OSiEt3)4, Zr[OSi(OtBu)3]4, Hf[OSi(OtBu)3]4, (thd)2Zr(OSiMe3)2, and (thd)2Hf(OSiMe3)2.
Chemical vapor deposition of these single source metal siloxides, although simplifying some aspects in the production of thin films introduce different problems to the process and in the design of the thin-film composition. Zr(OSiMe3)4, for example, is a solid at room temperature. The compounds (thd)2Zr(OSiMe3)2 and (thd)2Hf(OSiMe3)2 are solids and suffer from low volatility due to the use of bulky thd ligands. Other precursors such as Zr[OSi(OtBu)3]4 and Hf[OSi(OtBu)3]4 also have been found to have relatively low volatility.
Single source metal siloxide precursors which are liquids such as Zr(OSiEt3)4 and Hf(OSiEt3)4, are preferred for thin-film applications. Although, in liquid form, they too can present problems when designing the desired composition of the resultant thin-film. The design becomes a problem because each of the previous liquid metal siloxides has a well-designed metal:Si ratio of either 1:4 (0.25) or 1:2 (0.5). For example Zr(OSiMe3)4 has a metal:Si ratio of 1:4 and (thd)2Zr(OSiMe3)2 has a metal:Si ratio of 1:2. To achieve thin-film compositional ratios other than that produced from the pure 1:4 and 1:2 complexes, the art has employed the use of multiple sources. One technique employing secondary sources for achieving metal silicon ratios other than the fixed ratios of 1:2 and 1:4. is through the use of mixture of a silicon containing metal (zirconium) source. e.g., Zr(OSiMe3)4 and a non-silicon containing metal (zirconium) source, e.g., Zr(OtBu)4 delivered either separately, as a mixture or dissolved in a solvent. Another technique is through the use of a non-metal containing compound of silicon, e.g., Si(NMe2)4 and a metal siloxide compound, e.g., Zr(OSiMe3)4 delivered either separately, as a mixture or dissolved in a solvent. Another technique uses a non-metal containing silicon compound, e.g., Si(NMe2)4 and a metal containing compound, e.g., Zr(NEt2)4 delivered either separately, as a mixture, or dissolved in a solvent.
Single source mixtures of separate complexes introduce a number of problems to the CVD process. Because the relative rate of deposition for each precursor may vary significantly with temperature and pressure due to differences in activation energy, it is difficult to uniformly control the metal:Si ratio deposited as a thin-film dielectric. The use of separate metal and silicon sources also requires delivery equipment for multiple chemical sources as well as control over the ratio in which they are delivered.
Representative patents and articles illustrating the preparation and deposition methods for single source precursors are as follows:
WO 01/25502 discloses gate dielectric films of zirconium and hafnium silicates on silicon substrates. The precursor metal compound includes a metal such as zirconium or hafnium and at least an alkoxide or β-diketonate; a second precursor not containing silicon to provide a ratio Mx/Si1- x of from 0.01 to 0.99. These precursors are produced by reacting a β-diketone, such as 2,2,6,6-tetramethyl-3,5-heptanedione (Hthd), with ZrCl2 suspended in diethyl ether. The resulting compound then are mixed in one to two molar ratio with LiOSiMe3, thereby forming Zr(thd)2(OSiMe)2. Deposition of a mixture of Zr(OSiMe3)4 and Zr(OtBu)4 is shown.
U.S. Pat. No. 6,238,734 discloses the deposition of a metal compound or mixture of at least a two ligand complex onto substrates suited for semiconductor fabrication. A mixture of two or more metal-ligand complexes as the precursor is employed. Ligands are the same and are selected from the group consisting of alkyls, alkoxides, halides, hydrides, amides, imides, azides, nitrates, cyclopentadienyls, carbonyls, and their fluorine, oxygen and nitrogen substituted analogs. Representative metal-ligand complexes include: Si(N(CH2CH3)2), Ti(N(CH2CH3)2)4, Zr(N(CH2CH3)2)4, Hf(N(CH2CH3)2)4, V(N(CH2CH3)2)5, V(N(CH2CH3)2)4, Nb(N(CH2CH3)2)5, Nb(N(CH2CH3)2)4, CH3CH2N═Nb(N(CH2CH3)2)3, CH3CH2N═V(N(CH2CH3)2)3, (CH3CH2N═)2W(N(CH2CH3)2)2, (CH3CH2N═)2Mo(N(CH2CH3)2)2, and CH3CH2N═Ta(N(CH2CH3)2)3.
Terry et al in an article, Trialkoxysiloxy Complexes as Precursors to MO2.4SiO2(M═Ti, Zr, Hf) Materials, Chem. Mater. 1991, 3 1001–1003 disclose chemical routes to ceramic materials based upon alkoxysiloxy transition-metal complexes as single source precursors to homogenous metal silicate networks. These complexes of the formula M[OSi(OtBu)3]4 where M═Ti, Zr, or Hf are used to produce MO2.4SiO2 materials. One type of complex is produced by refluxing a toluene solution of HOSi(OtBu)3(4 equiv) with Ti(NEt2)4.